1. Field of the Invention
This invention relates to liquid crystalline compounds and mixtures.
2. Description
Liquid crystals have recently gained considerably importance primarily as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well known to the person skilled in the art and can be based on various effects such as, for example, dynamic scattering, the deformation of aligned phases (DAP type), the Schadt-Helfrich effect (rotation cell), the guest/host effect or a cholesteric-nematic phase transition (phase change effect).
Liquid crystals must satisfy a number of requirements in order to be suitable as dielectrics for electro-optical indicating devices. For example, they must have a good chemical stability towards environmental factors such as, for example, heat, moisture, air and electromagnetic radiation in the infrared, visible and ultraviolet range. Further, they should be colourless, should have short response times and should have not too high a viscosity, should give a good contrast and should have a nematic or cholesteric mesophase in the entire temperature range in which the liquid crystal cell is to be operated. Other properties must fulfil different conditions depending on the type of cell which is used; for example, liquid crystals which are used in rotation cells should have a large positive anisotropy of the dielectric constants (.DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp. &gt;0, .epsilon..sub..parallel. signifying the dielectric constant along the longitudinal molecular axis and .epsilon..sub..perp. signifying the dielectric constant perpendicular thereto) and liquid crystals which are used in guest/host cells should have a large positive or negative anisotropy of the dielectric constants. Moreover, in both cases a low threshold potential and a conductivity which is as small as possible are desirable.
Since, in general, it is not possible to achieve all desired and to some extent contradictory properties with a single compound, attempts have mainly been made to optimize the properties for the particular applications by mixing several components. In this case it is, however, important that the components undergo no chemical reactions with one another and have a good miscibility. Further, the mixtures formed should have no smectic mesophases, at least at temperatures at which the liquid crystal cell is to be operated.
There are already known a number of liquid crystalline compounds and doping agents for liquid crystal mixtures which have as wing groups, for example, alkyl, alkoxy, alkanoyloxy, alkoxycarbonyl and cyano groups.